In the majority of electronic products we see, an extremely important component is indispensable: the diode. At first glance, a diode may just be a small silicon wafer with two leads, but its function goes far beyond that. It is like a "one-way gate" for electric current, ensuring that the current can only flow in one direction. This simple yet crucial ability has led engineers to call it one of the most fundamental building blocks in electronics.
In fact, there are many types of diodes, which are designed for different tasks in circuits. In this comprehensive guide, we will explain what a diode is, show diode symbols and diode diagrams, and explain step by step how a diode works. Most importantly, we will delve into the most important types of diodes in modern electronics to help you clearly understand their functions and practical applications.
What Is a Diode?
First of all, what is a diode? In simple terms, a diode is a two-terminal semiconductor device. Its most distinctive feature is that it only allows current to flow in one direction and prevents it from passing in the opposite direction. It is precisely because of this characteristic that it has become one of the most fundamental and common components in electronics.
A diode has two terminals:
• Anode of diode - the positive terminal
• Cathode of the diode - the negative terminal
Whether current can pass or not depends on the diode polarity. If the diode anode and cathode are properly connected to the power supply, the current will flow smoothly. If they are connected in reverse, the diode will block the current.
In a circuit diagram, the diode symbol is usually drawn as a triangle pointing to a vertical line. The triangle represents the anode of the diode, and the vertical line represents the diode cathode. This simple diode diagram is easy to remember and also facilitates engineers to quickly confirm the direction when discussing circuits. In daily life, people also directly look at these diode symbols to judge the flow of current.
Although diodes are small, they have a wide range of applications. As long as you understand what a diode is, its structure, diode polarity, and various types of diodes, you can lay a solid foundation for learning and mastering more complex electronic circuits.
How Do Diodes Work?
To figure out how a diode works, let's start with the structure of a diode. Most types of diodes are made based on a PN junction. A PN junction is actually a combination of two different semiconductor materials:
• P-type semiconductor: It contains many "holes" inside, which are places lacking electrons, and can be regarded as positively charged carriers.
• N-type semiconductor: It has extra electrons inside, that is, negatively charged carriers.
When the P-type and N-type materials are combined, electrons and holes will meet at the junction and cancel each other, forming a region without free charge carriers, which is the depletion region. It is like a "wall", preventing electric current from passing through freely.
Biasing Modes of a Diode
The performance of a diode in a circuit depends on the diode polarity, that is, how the diode anode and cathode are connected to the power supply:
• Forward bias: If the anode of the diode is connected to the positive terminal of the power supply and the diode cathode is connected to the negative terminal, the depletion region will become narrow. When the voltage exceeds a certain value - called threshold voltage, approximately 0.7V for silicon diodes and 0.3V for germanium diodes, current can flow through.
• Reverse bias: If the polarity is reversed, the depletion region will widen, and the current will hardly pass through, leaving only a very small leakage current.
• Zero bias: The diode remains non-conductive when no voltage is applied.
In simple terms, a diode is like a one-way street. If the current enters from the correct direction (forward bias), it can pass through smoothly. If the current attempts to enter from the opposite direction (reverse bias), it will be blocked by the "wall" of the depletion region.
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Common Types of Diodes
Now let's take a look at the most common types of diodes. Although all diode types have the basic function of unidirectional conduction, they differ in design structure and characteristics, and thus, their application scenarios are completely different. The following introduces several different types of diodes one by one, including their characteristics, advantages and applications.
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Diode Type
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Description
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Key Features
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Applications
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Rectifier Diode
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Basic type of diode used for power conversion.
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Converts AC to DC; handles high current and voltage.
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Power supplies, adapters, chargers
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Small Signal Diode
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Designed for small currents and voltages.
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High-speed switching, low capacitance.
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RF circuits, logic switching
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Zener Diode
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Special diode working in reverse breakdown.
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Maintains constant Zener voltage; voltage regulation.
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Power stabilization, surge protection
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Schottky Diode
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Metal-semiconductor junction diode.
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Low forward voltage drop (0.2–0.4V), very fast switching.
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Power supplies, RF circuits, solar panels
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Light Emitting Diode (LED)
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Emits light when forward-biased.
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Energy-efficient, long lifespan, multi-color options.
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Displays, lighting, indicators, automotive
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Photodiode
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Converts light into electrical current.
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High sensitivity, fast response, reverse-biased operation.
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Solar cells, optical communication, sensors
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Laser Diode
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Similar to LED but produces laser light.
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Emits a coherent, monochromatic beam.
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Fiber optics, barcode scanners, DVD/CD drives
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Avalanche Diode
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Operates in the avalanche breakdown region.
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Withstands high voltages; absorbs surges.
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Surge protection, high-voltage circuits
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TVS Diode
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Transient Voltage Suppression device.
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Very fast response; clamps voltage spikes.
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ESD protection, communication lines
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PIN Diode
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P-layer, intrinsic layer, N-layer structure.
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Variable resistor at high frequency.
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RF switches, attenuators, photodetectors
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Varactor (Varicap) Diode
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Acts as a voltage-controlled capacitor.
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Capacitance varies with reverse voltage.
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RF tuning, PLL circuits, VCOs
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Tunnel Diode (Esaki)
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Heavily doped PN junction diode.
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Tunneling effect, negative resistance.
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Microwave oscillators, amplifiers
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Backward Diode
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Variant of tunnel diode for small signals.
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Optimized for low-voltage RF signals.
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RF detection, signal rectification
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Step Recovery Diode (SRD)
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“Snap-off” diode for sharp pulses.
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Generates fast rise-time signals.
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Frequency multipliers, radar systems
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Gunn Diode
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Made from N-type material, no PN junction.
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Uses Gunn effect to generate microwaves.
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Radar, microwave oscillators
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Gold Doped Diode
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PN diode doped with gold atoms.
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Short carrier lifetime, very fast switching.
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High-speed detection circuits
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Constant Current Diode
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Current-limiting semiconductor device.
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Maintains constant current flow.
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LED drivers, battery chargers
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Peltier / Thermal Diode
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Moves heat instead of current.
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Based on Peltier effect; directional heat transfer.
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CPU cooling, thermal management
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Vacuum Diode
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Early diodes were built in vacuum tubes.
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Thermionic emission, high voltage capability.
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Early radios, old power supplies
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Applications of Different Types of Diodes
Different types of diodes not only differ in structure, but more importantly, they are applied in various scenarios in actual circuits. From power rectification and voltage regulation to signal processing, optoelectronic devices and circuit protection, each type of diode has its unique role. The following table summarizes the main application categories of diodes and the corresponding typical diode types.
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Category
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Applications
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Common Diode Types
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Power & Rectification
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• AC to DC conversion in power supplies
• Bridge rectifiers
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Rectifier diodes
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Voltage Regulation & Clamping
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• Voltage regulation
• Clamping surges
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Zener diodes, TVS diodes
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Signal Processing
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• RF circuits
• Generating sharp pulses
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Tunnel diodes, PIN diodes, Step Recovery Diodes (SRDs)
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Optoelectronics
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• Lighting and displays
• Optical communication systems
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LEDs, Photodiodes, Laser diodes
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RF & Microwave
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• Microwave oscillators
• Frequency tuning
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Gunn diodes, Varactor diodes
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Protection Circuits
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• Inductive load protection
• ESD suppression
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Flyback diodes, TVS diodes
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Thermal & Legacy Applications
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• Cooling processors
• Early electronics
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Peltier diodes, Vacuum diodes
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Choosing the Right Diode
When faced with multiple diode types, ask these questions:
1. What is the purpose? (rectify, regulate, detect, emit, protect)
2. What voltage and current ratings are needed?
3. Is high speed required? Schottky or PIN may be best.
4. Is efficiency critical? Look at the forward voltage drop.
5. Will temperature affect performance? Check the datasheet.
Conclusion
There are many types of diodes, ranging from the most common rectifier diodes to specialized laser diodes and Gunn diodes, all of which demonstrate the versatility of these small components. As long as you understand what a diode is, its working principle, circuit symbols, polarity, and the characteristics of different types of diodes, you can make the right choice more confidently when designing or analyzing circuits.
Frequently Asked Questions (FAQs)
Q1: What is the most common diode?
A1: The rectifier diode is the most common, widely used for converting AC to DC.
Q2: Which diode is best for voltage regulation?
A2: The Zener diode is specifically designed to regulate voltage in reverse bias.
Q3: Why use a Schottky instead of a normal rectifier?
A3: Because the Schottky diode has a much lower forward voltage drop and switches faster.
Q4: Can LEDs be used as photodiodes?
A4: Yes, an LED can act as a photodiode when reverse-biased, though not as efficiently as dedicated photodiodes.
Q5: Is a Gunn diode really a diode without a PN junction?
A5: Yes. The Gunn diode uses only N-type material and relies on the Gunn effect instead of a PN junction, making it unique among diode types.
